The present invention relates to the field of electronic transmitters and more specifically to magnetron transmitters. Previously, magnetron transmitters have not been used for high resolution, short pulse radars because of their inherently slow starting characteristic.
In operation, the magnetron oscillates. To achieve proper initiation of oscillation the pulse applied to the magnetron by the modulator in the circuit must have a relatively slow rate of rise in the transition region, i.e., the region in which the magnetron begins to oscillate. If the applied pulse rises too rapidly in the critical region either of two types of misfire may occur. In the first type the tube may go into oscillation in the wrong mode, i.e., at the wrong frequency and at a higher voltage. In the second, the tube may not oscillate at all. Both of the above misfires may be accompanied by a high voltage arc-over inside the tube.
The requirement for a slowly rising modulator pulse is incompatible with the generation of a very short pulse. Using conventional techniques it has never been possible to operate a magnetron with pulses as short as 25 nanoseconds, and for this reason pulse compression systems have been primarily used to obtain high range resolution.
At present, most high resolution radar systems use pulse compression techniques. In that approach long pulses on the order of a microsecond of linearly swept radio frequency are generated using a voltage controlled oscillator. These pulses are amplified and transmitted. In some systems high range resolution is obtained by time-compressing the received signals in a frequency dispersive delay network.
Other systems (frequency domain pulse compression) use an array of band-pass filters to separate the signals from different ranges, and in one pulse period data are available only from narrow strips of the overall range. In both of these systems high range resolution is bought at the cost of complexity and considerable economic expense.
High range resolution could be obtained using a simple, conventional radar system if a short pulse, high power transmitter were available. None have been in the past. These requirements are met for the first time by the short pulse, magnetron transmitter of the present invention. Its advantages over the pulse compression systems are summarized below.
1. Simplicity. No frequency modulated r-f signal generator and accompaning waveform generators are needed. No power amplifiers are needed. The transmitted signal is generated in one device at the output power level. There are no images (as in the frequency domain, pulse compression system) and no image rejection schemes are necessary. No pulse compression network nor band-pass filters are needed.
2. Low Cost. The relative simplicity results in a considerable saving. Also, a magnetron is a low-cost device and is much cheaper than the traveling wave tube amplifiers used in the pulse comppression systems.
3. Performance. The radar using a short pulse magnetron transmitter should have performance equal to that of either type of pulse compression system. It will have an advantage over the frequency domain system in that data are available from all range increments in each pulse period rather than only from narrow strips.
4. Short Range Capability. A radar system is blindto targets at ranges less than (.delta./2) feet, where .delta. is the pulse duration in nanoseconds. A short pulse system will be able to see targets at much closer ranges, e.g., 6 feet, than the pulse compression systems. The latter will be blind to targets closer than the order of 500 feet.